When engineers specify an AODD pump for aggressive chemical service, casing material gets most of the attention. PVDF for strong acids. PP for bleach and caustic. Acetal for solvents. The selection logic is sound — but it focuses on the wrong component. In the majority of chemical pump failures, the diaphragm is what fails first, not the casing, and the failure modes are different and more damaging.
This matters because a casing failure is usually a slow leak you can see and respond to. A diaphragm failure puts the process chemical directly into the pump’s air system — and potentially into the compressed air supply for the rest of your facility.
Why the Diaphragm Has a Harder Job Than the Casing
The casing experiences chemical contact under relatively static conditions. It holds fluid, it sees temperature and pressure, but it does not move. The diaphragm experiences all of that simultaneously with repeated mechanical flexing — typically hundreds of thousands of cycles over its service life.
This combination of chemical attack and mechanical stress is fundamentally different from what the casing sees. A material that is chemically resistant but mechanically brittle will crack under flex cycling even if it never corrodes. A material that is flexible but has marginal chemical resistance will swell, harden, or lose tensile strength from chemical absorption — and then fail under the mechanical load it was previously able to tolerate.
PVDF casing and PP casing are rigid. They do not flex. The chemical compatibility data you use to select them describes resistance under static immersion conditions — which is appropriate for a casing. That same data applied to a diaphragm is only part of the picture.
The Four Main Diaphragm Materials and What They Handle
PTFE is the broadest-spectrum diaphragm material available. It resists virtually all acids, alkalis, solvents, and oxidisers at concentrations up to the material’s rated temperature limit. For sulphuric acid, hydrochloric acid, sodium hypochlorite, hydrogen peroxide, nitric acid, and most organic solvents, PTFE is the default choice. Its weakness is mechanical: PTFE is not elastic. It cold-creeps under sustained load, takes a permanent set over time, and accumulates fatigue damage faster than elastomeric materials under high-cycle conditions. PTFE diaphragms in demanding applications should be inspected on a shorter cycle than the manufacturer’s general recommendation if fluid specific gravity is above 1.3 or stroke rates are high.
EPDM handles aqueous solutions well — water, mild acids, alkalis, and many water-treatment chemicals including chlorine dioxide and sodium hypochlorite at moderate concentrations. It is flexible and recovers elastically, giving it good fatigue resistance compared to PTFE. Its critical limitation is hydrocarbons: EPDM swells rapidly in contact with oils, fuels, aromatic solvents, and many chlorinated solvents. If your process fluid contains even trace hydrocarbons, EPDM is the wrong diaphragm material regardless of what the primary chemical is.
Santoprene (TPE) is a thermoplastic elastomer that combines flexibility close to EPDM with somewhat broader chemical compatibility. It handles many aqueous systems, mild acids, and some alcohols. It is not suitable for strong oxidising acids, concentrated sulphuric acid, or ketone and ester solvents. Santoprene’s main advantage is its flex life — it can outlast EPDM in high-cycle, moderate-chemistry applications. For food-grade and pharmaceutical transfer of non-aggressive aqueous products, it is a reasonable choice.
Hytrel (thermoplastic polyester elastomer) has the best flex life of the common diaphragm materials and reasonable resistance to hydrocarbons and fuels. It is frequently specified in fuel transfer, hydraulic fluid handling, and applications where mechanical endurance is the primary requirement. Its chemical compatibility with aggressive process chemicals — strong acids, oxidisers, ketones — is limited. Do not confuse its mechanical performance with chemical universality.
The Mismatch Problem
The most common diaphragm failure pattern in chemical applications is not extreme chemical attack — it is accumulated degradation from a material that is marginally incompatible with the fluid being pumped. The diaphragm swells slightly, losing dimensional accuracy and increasing the force required to flex it on each stroke. Or it hardens slightly, reducing elasticity and concentrating stress at the shaft hole. Either change accelerates the failure that was eventually going to occur anyway — but now it occurs within weeks instead of months.
This pattern is difficult to diagnose because the diaphragm looks intact when inspected visually. The dimensional and hardness changes are not visible. The only indicators are a shift in pump behaviour — higher air consumption for the same flow, or a change in stroke sound — and a service life shorter than expected.
If your diaphragms are failing significantly faster than the manufacturer’s estimated service life, the first thing to verify is that the diaphragm material is correctly matched to the actual process fluid, not the primary fluid alone. Cleaning agents, flush solvents, and secondary chemicals that contact the pump intermittently can cause material degradation even if the main process fluid is compatible.
Backing Diaphragms and Sandwich Construction
Some AODD pump configurations use a sandwich construction: a PTFE face diaphragm bonded to an elastomeric backing diaphragm. The PTFE face handles the chemical contact; the elastomeric backing provides the flex recovery that PTFE alone cannot. This construction is common in the Fluimac Phoenix range for demanding chemical applications.
When specifying a sandwich diaphragm, both materials need to be compatible with your application — not just the PTFE face. If the elastomeric backer contacts process fluid through a pinhole in the PTFE face (which happens with fatigue), an incompatible backer material will fail rapidly and unpredictably. Know what both layers are made of before finalising the specification.
A Practical Specification Approach
Start with the chemistry. Identify every fluid the diaphragm will contact — process fluid, cleaning chemicals, flush liquids, and any fluid that may enter through check valve leakage during maintenance. Cross-reference each against the diaphragm material’s compatibility data, not just the casing material table. Where you have doubt, PTFE is the conservative default for chemical resistance, but confirm that your application’s stroke rate and fluid specific gravity are within the range that PTFE fatigue life can support.
Then confirm the casing. A correctly specified diaphragm in a marginally specified casing is still a failure waiting to happen — but the failure mode is different, slower, and more visible. Get both right, in that order of priority.
If you are unsure which diaphragm material is correct for your application, the Autoflo team can help you work through the compatibility and fatigue life requirements. Contact us at info@autoflotechnology.com.